1. Field of the Invention
This invention relates generally to an oral composition which when applied onto the surface of teeth acts to whiten teeth and more particularly to an oral composition for whitening teeth that is more effective than existing products available to the consumer.
2. The Prior Art
A tooth is comprised of an inner dentin layer and an outer hard enamel layer that is the protective layer of the tooth. The enamel layer of a tooth is naturally an opaque white or slightly off-white color. It is this enamel layer that can become stained or discolored. The enamel layer of a tooth is composed of hydroxyapatite mineral crystals that create a somewhat porous surface. It is believed that this porous nature of the enamel layer is what allows staining agents and discoloring substances to permeate the enamel and discolor the tooth.
Many substances that an individual comes in contact with on a daily basis can "stain" or reduce the "whiteness" of one's teeth. In particular, foods, tobacco products and fluids such as tea and coffee that one consumes tend to stain the teeth. These products or substances tend to accumulate on the enamel layer of the tooth and form a pellicle film over the teeth. These staining and discoloring substances can then permeate the enamel layer.
One method for whitening teeth used by dental professionals involves the use of 30% hydrogen peroxide in combination with heat and light to promote the oxidation reaction. This method, although fast, is losing favor with dentists because clinical and scientific evidence shows that an effective whitening process without heat and light is desired.
Another professional method for bleaching teeth involves the use of hydrogen peroxide generating compounds such as urea peroxide (carbamide peroxide) at concentrations of 10% to achieve the desired whitening effect. Urea peroxide rapidly breaks down into hydrogen peroxide due to the water present in saliva. This method is known as an office-monitored at-home bleaching system and involves the use of a mouth guard or tray within which the bleaching agent is placed. The tray is then placed upon the teeth of the user and bleaching is allowed to take place. This method of treatment has drawbacks including tooth sensitivity, possibly due to demineralization and irritation of oral tissues. An additional disadvantage of the tray application method is that the bleaching effect is very slow.
There is a demand in the marketplace for a tooth whitening product that can be used at home or in private by the consumer and is safe and easy to use. A product for home use should not utilize the compositions or products for whitening teeth that are available for use by a trained dental professional. For example, the 30% hydrogen peroxide bleaching agent utilized by many dental practitioners to bleach teeth is sufficiently concentrated to be irritating and potentially dangerous for home use by the consumer.
More recently it has been discovered that peracetic acid is a surprisingly effective bleaching or whitening agent for discolored or stained human teeth, as disclosed in copending U.S. patent application Ser. No. 07/796,160, filed Nov. 22, 1991, the complete disclosure of which is incorporated herein by reference. As described in that application, 1% by weight aqueous solution of peracetic acid gives rise to a faster and superior whitening effect when applied to teeth at ambient to oral range temperatures than does a 30% by weight aqueous solution of hydrogen peroxide. As also described in the above referenced application, the peracetic acid can be applied directly to the teeth as by swab application, incorporated in an oral composition such as a toothpaste, gel or rinse that is to be applied topically, or generated in situ in the oral composition by the reaction of a peroxide source such as hydrogen peroxide, urea peroxide, sodium perborate, sodium percarbonate, and metal peroxides, for example, SrO.sub.2, CaO.sub.2 and NaO.sub.2, with peroxyacid precursor or activator containing labile acetyl groups. Illustrative examples of such activators include tetracetylethylenediamine, pentaacetylglucose, tetracetylglycoluril, sorbitol hexaacetate or fructose pentaacetate.
One of the major disadvantages associated with the use of peracetic acid packaged for home use by the consumer is its relative instability. Dilute 1% aqueous solutions of peracetic acid will substantially decompose in as little as 30 days at ambient temperatures. Storage at 3.degree. C. significantly improves stability but not to the extent required for the normal market age for a consumer or professional product. In addition, many common adjuvants present in consumer and professional products such as flavorants and other organic materials can rapidly react with peracetic acid, destroying both the adjuvants and the peracetic acid.
These factors tend to dictate that a preferred approach for the employment of peracetic acid chemistry in dentifrice applications is to generate the peracetic acid in-situ at the time of use. A source of hydrogen peroxide and a carboxylate derivative of acetic acid, such as an amide or an ester, may be mixed together in water at a pH high enough to generate sufficient concentration of perhydroxyl anion from the hydrogen peroxide. The perhydroxyl anion nucleophilically attacks the acetate derivative producing peracetic acid in accordance with the following reaction: ##STR1##
Analogous chemistry has been employed to generate hydrogen peroxide and perorganic acids in dry bleach and laundering compositions. For example, British Patent 836,988 discloses textile bleaching compositions containing an inorganic H.sub.2 O.sub.2 source such as sodium perborate or sodium percarbonate and an organic carboxylic acid ester such as esters of phenols or esters of mono - or disaccharides containing 3 or more ester groups.
The in-situ approach towards generating peracetic acid has also been disclosed with respect to dentifrice compositions as described in U.S. patent application Ser. No. 07/796,160, referred to above.
In addition, U.S. Pat. No. 5,055,305 discloses effervescent tablets for the in vitro cleaning of dentures which contain, as essential components, a bleaching agent which comprises salts of persulfate perborate or pyrophosphate hydrates or metal peroxides, a peroxyacid bleach precursor and an effervescence--producing base composition. Among the numerous organic peracid precursors disclosed are carboxylic acid esters such as acetylsalicylic acid, which are more generally disclosed in the aforementioned British Patent 836,988, directed towards textile bleaching and detergent compositions.
In those applications where dentifrice compositions are designed for in-vivo use, it is essential that the peracetic acid generating components react quickly after they are combined, since the user will normally wish to limit the time in which the dentifrice is in contact with the teeth. In addition, the classes of peroxide generators and peroxy acid bleach precursors useful for in-vivo application to the teeth is severely limited due to the requirement that these components by physiologically safe and non-irritating to oral tissues. A further requirement for in-vivo use is that the peracetic acid is generated at a relatively neutral pH, close to the safe physiological neutral pH of 7.